Howard Butler sums up my motivations pretty well. The one factor he doesn't mention: community. There's a gap between the GDAL/OGR/MapServer/OSGeo and Python communities, which I jumped a while ago. Check out the list of GIS projects in the Python community's package index: Topic :: Scientific/Engineering :: GIS. There's not one OSGeo package there now; Howard's Python bindings for GDAL 1.5 will be the first. The GIS community is an inward-looking one, and the open source side of it looks outward only fractionally more.

Update: It's also true that the GDAL community's bindings aren't dead yet. As I pointed out previously, the new style bindings (to be standard in 1.5) are so much faster than the original bindings that everybody's a winner whether they switch to WorldMill or not.